Histology of the Eye (Part 1) PDF

Title: Histology of the eye Present By: Dr.Doaa Khaled, lecturer of histology & cytology OBJECTIVES 1- Mention components of eye ball & eye compartments 2- Describe histological structure of cornea. 3- Mention why the cornea is transparent? 4- Describe histological structure of middle vascular coa...

Title: Histology of the eye Present By: Dr.Doaa Khaled, lecturer of histology & cytology OBJECTIVES 1- Mention components of eye ball & eye compartments 2- Describe histological structure of cornea. 3- Mention why the cornea is transparent? 4- Describe histological structure of middle vascular coat The eye The eye is a photoreceptor organ. It is found in a bony cavity in the skull which is called the orbit. The wall of the eye ball is formed of 3 layers: I) The outer layer: It is a fibrous coat, formed of cornea anteriorly and sclera posteriorly. II) The middle layer: It is the vascular coat, formed of three parts 1-The iris. 2-The ciliary body. 3-The choroid. III) The Inner layer: It is the nervous coat, formed of the retina The eye contains three compartments: 1-The anterior chamber: between the cornea and the iris. 2-The posterior chamber: between the iris, ciliary body and the lens. 3- The vitreous space: behind the lens. Ø The anterior and the posterior chambers are connected by small opening in the iris called the pupil. Ø Both chambers contain protein-poor fluid called aqueous humor, while the vitreous space is filled with a gelatinous material called vitreous body. I) The outer fibrous coat ( the cornea) 1-The epithelium stratified squamous non- keratinized epithelium, formed of the following layers: a-The basal cell layer: one layer of cubical cells rest on a straight basement membrane. b-The intermediate layers: Polygonal cells connected by desmosomes. C-The top layer: Ø flat squamous epithelium. Their free borders contain microvilli which retain a film of tears keep the surface Wet The corneal epithelium is very sensitive as it is richly supplied by free nerve endings ØThe epithelium showed also high rate of regeneration from the basal layer. 2)- Bowman’s membrane Ø non cellular transparent homogenous membrane. Ø formed of a network of collagen & intercellular substances. Ø It acts as a protective layer against any trauma or bacterial invasion. ØInjury scar formation corneal opacity 3-Substantia propria Ø It forms the main thickness of the cornea (about 90%) (formed of 200 lamellae of collagen type I) with large amount of ground substance & fibroblasts (Corneal corpuscles) in-between ØThe collagen fibrils are arranged parallel to the corneal surface. 4-Descemet’s membrane It is non cellular homogenous membrane. Formed of irregularly arranged collagen fibrils. 5-Descemet's endothelium - one layer of flat squamous cells that lines the posterior surface of the cornea. - highly rich in mitochondria and microvesicles. Function: They actively absorb water from the substantia propria to the aqueous humor Why the cornea is transparent? 1- The cornea is avascular structure ( not penetrated by blood vessels). So the cornea takes its nutrition from the aqueous humor and from the scleral capillaries in the corneo-scleral junction. 2- Regular arrangement of collagen lamellae. 3-The same refractive index of the collagen fibrils and the ground substance. 4-Continuous withdrawal of water from the cornea by Descemet’s endothelial cells. lamina cribrosa. B) The sclera: The sclera is the opaque white posterior part of the outer fibrous coat of the eye ball. Its opacity is due to: a- irregular arrangement of its collagen fibers b- its relative high content of water. The sclera is formed of three layers: Sclera proper is formed of dense irregular connective tissue. (white fibrous C.T.) The posterior part of the sclera which is perforated by the optic nerve is called lamina cribrosa. II) The middle vascular coat Also called uveal tract. It is formed of 3 parts, the choroid, the ciliary body and the iris a-The choroid: - It is the posterior part of the vascular coat, - Separated from the sclera by the perichoroidal space. b-The ciliary body: - it appears triangular in shape. - Its apex is continuous posteriorly with the choroid and its base faces the iris. - The ciliary processes project from its medial border. I - t is covered by two layers of cubical cells, the inner layer is pigmented. The ciliary body: It is formed 1) Loose connective tissue: highly rich in elastic fibers and melanocytes. 2) Ciliary muscles - Smooth muscle fibers arranged in two bundles redial and circular fibers. - Contraction and relaxation of these muscles can change in the shape of the lens which is important in visual accommodation. 3) Ciliary processes - These are projection from the medial side of the ciliary body. - They are formed of loose connective tissue a+ fenestrated blood capillaries, - covered by two layers of cubical cells as the ciliary body. Function: 1-Secretion of the aqueous humor 2-Give the attachment to the Suspensory ligament of the lens (Zonule) C-The iris - It is the anterior part of the middle coat: 1) Anterior stromal layer (pigment cells + fibroblasts) 2) Posterior vessel layer, (more vascular) 3) Loose connective tissue (muscle of the iris): a)The sphincter pupillae muscle: a circular band that causes constriction of the pupil. b)The dilator pupillae muscle radial smooth ms causes dilatation of the pupil. 4) Posterior surface is covered by two layers of epithelial cells as that c over the ciliary body. The inner one is posterior pigmented. Function of the iris 1-It gives the colour of the eye. 2-The pupillae muscle controls the size of the pupil. 3-Heavy pigmentation of the epithelium prevents passage of light into the interior of the eye except through the pupil. Title: The eye (PART 2) Present By: Dr.Doaa Khaled, lecturer of histology & cytology OBJECTIVES 1- Mention structure of eye lens 2- Describe histological structure of retina. 3- Compare between rods & cons 4- Define components of refractive media of the eye 5- Define fovea centralis 4- Describe histological structure of the conjunctiva & Eye lid The lens The lens is a transparent biconvex structure. attached to the ciliary body by a ligament called zonule. It has 3 components: 1-Lens capsule : which is rich in collagen and glycoprotein. 2-Subcapsular epithelium - formed of single layer of cubical cells on the anterior surface of the lens. 3-The lens fibers - originated from the epithelial cells that lose their nuclei and organoids. - They are elongated fibers and contain crystalline protein. The vitreous body It is a gel non vascular mass that contains water, collagen and hyaluronic acid. It occupies the interior of the eye ball between the lens and retina. It is traversed by the hyaloid canal. Function 1-Transmission of light to the retina. 2-Supports the lens and the retina. 3-Allows transfer of materials from and to the retina. Refractive media of the eye There are four transparent components through which the light passes till it reaches the retina. 1-The cornea. The most effective one. 2-The aqueous humor. 3-The lens. 4-The vitreous body. III) The inner nervous coat (The retina The retina is formed of 10 layers: 1-The pigmented epithelium 2- Rods& Cons 3-The outer limiting membrane. 4- Outer nuclear layer 5- The outer plexiform layer 6- The inner nuclear layer 7- The inner plexiform layer 8- The ganglion cell layer 9) The optic nerve layer 10) The inner limiting membrane III) The inner nervous coat (The retina The retina is formed of ten layers: 1)The pigmented epithelium Single layer of columnar epithelium with basal nuclei, comes in contact with the Bruch’s membrane of the choroid. The cell membrane of these cells shows 1-The lateral surface is connected to adjacent cell by junctional complex and gap junction. 2-The apical border shows microvilli that envelop the tips of the photoreceptors. 3-The basal border shows basal infoldings with numerous mitochondria. Function 1-Absorption of extra light to prevent blurring of vision. 2-Phagocytosis of the old disks detached from the tips of rods and cones. 3-Formation of vitamin A which passes to the rods and cones. 4- blood retinal barrier. 2) The layer of rods and cones it is formed of the outer photosensitive segments of both rods and cones. 3)The outer limiting membrane. it is formed by the junctional complex between the outer processes of the Muller’s cells + the adjoining rods and cones. 4) The outer nuclear layer - Consists of: cell bodies of both rods and cones, 5) The outer plexiform layer formed by synaptic connection between the synaptic bodies of rods & cones with the dendrites of bipolar nerve cells and the processes of the horizontal cells. 6) The inner nuclear layer: It contains the nuclei of: A- The bipolar nerve cells: Two types of bipolar cells B-Horizontal cells C-Amacrine cells D-Muller cells - They are supporting cells with long cytoplasmic processes. - The outer processes extend externally to form the outer limiting membrane, while the inner processes extend internally to form the inner limiting membrane. 7) The inner plexiform layer reticular layer Synaptic connection between the axons of the bipolar nerve cells and the dendrites of the ganglion cells+ The processes of the Amacrine cells synapse with these two cells. 8) The ganglion cell layer It is formed of large nerve cells with vesicular rounded, central nuclei. Their dendrites synapse with the axons of bipolar cells, while their axons form the optic nerve. There are two types: a-Diffuse ganglion cells that synapse with rod bipolar cells. b-Midget ganglion cells that synapse with one midget cell. 9) The optic nerve layer It is formed of axons of the ganglion cells that pass at the right angle to form the optic nerve. 10) The inner limiting membrane It is formed of the inner processes of the Muller cells lying on the basement membrane. Rod and cone cells: These are photoreceptors. Both have the same structure with some differences. - Rods mediate dim light vision. - about 120 million of rods in each human retina. - The rod cell is formed of (outer and an inner segments) connected by a narrow region called the connecting stalk. 1) The outer segment: - It is formed of transversally parallel membranous disks. - These disks are not continuous with the surrounding cell membrane. - The disks contain visual pigment (Rhodopsian). The connecting stalk contains a modified cilium showing nine peripheral doublets of microtubules with no central pair of microtubules, arising from a basal body in the inner segment. 2) The inner segment: - It is the protein forming part. - The outer part of the inner segment contains organelle of protein synthesis. The basal part of the inner segment contains: The nucleus appears small rounded and dark. Synaptic region : which is in the form of spherule that contains synaptic vesicles and mitochondria. Function of the rod cells: 1-The inner segment synthesizes the protein opsin which passes to the outer segment to be linked with retinene to form rhodopsin (visual pigments) in the disks. 2-The rods are stimulated by dim light which breaks down the rhodopsin into opsin and retinene causing hyperpolarization of their cell membrane that initiates photic impulses. The cone cells - They mediate day light vision. - Each human retina contains about 6 million cone cells. Each cell has the same structure of the rod cell except the following differences: 1)The outer segment of cone is conical in shape with 2)The membranous disks are continuous with the cell membrane. 3) The membranous disks contain iodopsin pigments which are sensitive to colour vision blue, green and red light. 4) The cone nuclei 5)Synaptic region is called pedicle that synapse with a single bipolar nerve cell in the outer plexiform layer Fovea centralis: It is a rounded depression in the posterior pole of the eye, which is characterized by: 1-The layers from the fourth layer inwards displaced laterally. 2-Photoreceptors are of the cones only that are thinner and closer. 3-It has a high visual acuity The blood- retinal barriers 1) The outermost barrier - formed of tight junction between the pigmented epithelium of the retina. - It prevents the escape of the foreign bodies from the capillaries of the choroid to the outer part of the retina. 2) The innermost barrier: is formed by the tight junction between the endothelial cells of the retinal blood capillaries. Accessory structures of the eye (1-The conjunctiva) a) The bulbar conjunctiva: - That covers the anterior part of the sclera up to the cornea. - It is formed of stratified squamous non- keratinized epithelium + underlying loose connective tissue. b) The Palpebral conjunctiva It covers the inner surface of the eye lid. It is formed of stratified columnar epithelium with goblet cells. Function: Lubrication and protection of the eye by the mucous secretion of the goblet cells. 2) The eye lids:. It is formed of: 1)Thin skin : covers the anterior surface of the eye lid, with underlying loose connective tissue. No subcutaneous fat. 2) Skeletal muscle: orbicularis oculi muscle that closes the eye. 3) The tarsal plate which is formed of dense fibrous connective tissue containing modified sebaceous gland called Meibomian gland that opens in the free margin of the lid. 4) The palpebral conjunctiva. The free margin of the eye lid contains 3-4 rows of stiff hairs called eye lashes. These lashes are provided with modified sebaceous glands (glands of Zeis) and sweat glands (glands of Moll). 3) The lacrimal gland: (reading only) - They are two compound tubule-alveolar glands present in the superiolateral part of the orbit. - Each gland is formed of serous secretory acini that are lined with pyramidal cells, supported by myoepithelial cells. - They secrete tears by small ducts into the superior fornix of the eye. The tears are watery secretion that moistens the cornea and the conjunctiva. Drainage of the tears The tears are drained by two canliculi into the lacrimal sac, ---- a nasolacrimal tube ---- the nasal cavity. Both the lacrimal sac and the nasolacrimal tube are lined by two layers of columnar cells with goblet cells.